Pre- and Postharvest Calcium Treatment of Apple Fruit and its Effect on Quality

Calcium is thought to be the most important mineral element determining fruit quality. It seems to be especially important in apples where it has been shown to reduce metabolic disorders. Calcium in adequate amounts helps to maintain apple fruit firmness and decreases the incidence of physiological disorders such as water core, bitter pit and internal breakdown. Postharvest decay may also be reduced by increasing the calcium content of apple fruit. The major problem is getting enough calcium into the fruit to have the desired effects. Soil treatments with calcium to increase fruit calcium concentration have often met with very little success. The direct application of calcium to the fruit is the most effective method for increasing fruit calcium content. This can be accomplished by preharvest sprays or postharvest dips or vacuum or pressure infiltration. Although calcium sprayed on apples on the tree can increase calcium content of the fruit, direct postharvest treatment of the fruit in solutions of calcium chloride can be even more effective. Pressure infiltration with solutions of calcium chloride can increase the calcium concentrations of apple fruit more effectively than vacuum infiltration which is more effective than dipping. Both preand postharvest calcium treatment methods have inherent problems. Inadequate calcium uptake is a problem in some cases in that not enough calcium is taken into the fruit to have a positive effect. In other cases, excessive calcium is taken into the fruit and injury results. Developing a commercially acceptable method of successfully increasing calcium concentration in fruit is a continuing challenge. In the meantime, orchard management to optimize fruit calcium uptake in the preharvest environment combined with supplemental postharvest application is the best strategy to prevent losses due to low fruit calcium. INTRODUCTION Calcium (Ca) plays an extremely important role in plant growth and development and in maintaining and modulating various cell functions (Hanson, 1984; Palta, 1996). It is necessary to maintain membrane stability and is an integral part of the cell wall where it provides rigidity. It also has importance as an intracellular secondary messenger (Poovaiah and Reddy, 1993). Enhancing the calcium content of apple fruits can be very beneficial in maintaining fruit quality during storage. Calcium treatments can maintain fruit quality by reducing softening (Mason, et al., 1975), internal breakdown and respiration (Bangerth et al., 1972), bitter pit (Reid and Padfield, 1975), ethylene production (Sams and Conway, 1984 ), and decay (Conway and Sams, 1983). Postharvest losses due to Gloeosporium spp. were reduced in fruit sprayed preharvest with Ca sprays compared to unsprayed apples (Sharples and Johnson, 1977). Postharvest infiltration of CaCl2 solutions into apple fruit significantly reduced decay caused by Penicillium expansum Link, Botrytis cinerea Pers.:Fr., and Glomerella cingulata (Stoneman) Spauld. & Schrenk (Conway and Sams, 1983; Conway et al., 1991; Conway et al., 1992). The major problem is raising the Ca concentration of the fruit to a sufficient level to have the desired results. It has been Proc. IS on Foliar Nutrition Eds. M.Tagliavini et al. Acta Hort. 594, ISHS 2002 414 postulated that calcium tissue concentrations should exceed 250 μg g dry weight to control many calcium-related physiological disorders such as breakdown and bitter pit (Meheriuk and Moyls, 1989). In order to affect firmness or decay significantly, however, it is necessary to raise the level of tissue calcium to 800-1000 μg g dry weight (Sams and Conway, 1984; Conway and Sams, 1985). Concentrations significantly higher than 1000 μg g may cause surface injury to the fruit. Fertilization regimes, preharvest sprays, and postharvest treatment have all been tried in an effort to increase fruit tissue calcium in order to have a positive effect on fruit quality in storage. This paper will discuss some of the more recent literature on increasing the Ca concentration of apple fruit and the relationship this increased Ca has to physiological and particularly pathological maladies.